from The Century Dictionary and Cyclopedia

n. An attitude of vigilance; watch; guard: especially in the phrase on or upon the alert, upon the watch; on the lookout; guarding against surprise or danger: as, “the readiness of one on the alert,” Dickens.

HAMBURG, Germany — The Islamic militant whose disclosures under U.S. interrogation in Afghanistan triggered Europe's terror alert is an old friend of a man convicted in the 9/11 attacks and, as the strikes were being planned, frequented the same mosque where the Hamburg-based plotters often met, officials say.

In the aftermath of the arrest of terrorist suspect Farooque Ahmed and the revelation of an alleged plot to bomb multiple Metro stations, officials underscored the need for customers to remain alert to any suspicious activity, stressing that a single observation could prove essential.